[API Proposal]: add ToolStripItem.SelectedChanged event

[v-elnovikova]

Background and motivation

This is a follow-up to #3572

.NET Core 3.1 introduced a breaking change in using Menus. MenuItem control was removed and ToolStripMenuItem is recommended as a replacement.

However, the new ToolStripMenuItem class is missing the equivalent to MenuItem's Select event, that fires when menu item is highlighted by mouse or keyboard.

This event is most commonly used to display hints/descriptions/feedback to user when they navigate through menu items.

Some workarounds are the following ToolStripMenuItem's events: MouseEnter, MouseLeave, MouseHover, etc. But this does not cover the selection of a ToolStripMenuItem via keyboard (e.g. arrow keys).

Adding a new SelectedChanged event to the ToolStripItem class could be a solution to the problem. The event fires every time the Selected property value changes, similar to AvailableChanged or EnabledChanged events.

API Proposal

namespace System.Windows.Forms
{
    public class ToolStripItem
    {
        ...
        public event EventHandler? SelectedChanged;
        protected virtual void OnSelectedChanged(EventArgs e);
    }
}

API Usage

Here the SelectedChanged event is used to show a hint whenever user selects menu item with mouse or keyboard:

ToolStripMenuItem menuItem = new();
Label hintLabel = new();

menuItem.SelectedChanged += (object sender, EventArgs e) => 
{
    hintLabel.Text = menuItem.Selected ? "Menu item selected!" : string.Empty;
};

Alternative Designs

No response

Risks

No response

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[bartonjs]

Video

Looks good as proposed

namespace System.Windows.Forms
{
    public partial class ToolStripItem
    {
        public event EventHandler? SelectedChanged;
        protected virtual void OnSelectedChanged(EventArgs e);
    }
}

[Olina-Zhang]

Tested the fix in the latest .Net 9.0 SDK build: 9.0.100-alpha.1.23618.4, when using keyboard to focus or leave toolStripMenuItem, the SelectedChanged event is invoked once, but when using mouse to focus, the SelectedChanged event is invoked twice. @Epica3055 could you please look at it if the behavior is expected?

SelectedChanged_Test.mp4

[Epica3055]

@Tanya-Solyanik hi, I need to discuss this with you.

In framework, MenuItem have Select event. when the item's selection status turns from unselected to selected, the event will be fired. But when status turns from selected to unselected, the event won't be fired.

The question is what is expected behavior in .Net?

In the issue description, ToolStripItem.SelectedChanged , from semantic view, I think the event should be fired when the item's selection status turns from unselected to selected and from selected to unselected.

But that's not the case in Framework.

[Tanya-Solyanik]

@Epica3055 - what you are saying about symmetric behavior makes sense, if we were implementing a completely new event, we would had raised it in both cases - Selected->Unselected and Unselected -> Selected. However, the purpose of this implementation is to simplify migration from .NET Framework, so we should implement the same behaviour as the .NET Framework has.

[elachlan]

Closing issue as the PR and follow up have been merged.